The present invention relates to a new and improved construction of apparatus for the electrostatic application of coating material to articles, the coating material being present in the form of a liquid. The electrostatic coating apparatus of the invention is of the type comprising a nozzle arranged at the outlet opening of an infeed conduit formed of electrically insulating material, the nozzle serving for the atomization, without air, of the coating material which is under pressure. Further, there is provided an electrode arrangement to which there is applied a high-voltage for the electrostatic charging of the coating material and for maintaining an electrical field for the transport of the charged material particles to the article to be coated.
The unsatisfactory efficiency of older devices of this type wherein, for instance, there was arranged a rim of needle electrodes to which there was applied a high-voltage about the nozzle, could be improved, for instance as described in German patent 1,291,659 in that at the lengthwise side of a flat jet nozzle at the direct neighborhood of the spray jet there was provided a single electrode to which there was applied a high-voltage, especially an electrode in the form of a thin wire directed parallel to the spray jet axis. With this apparatus the grounded metallic handgrip serves as the counter electrode for the single high-voltage electrode. This metallic handgrip-- viewed in the coating or spraying direction--is located in front of the nozzle and has a relatively small spacing from the high-voltage electrode, so that the mean or average potential gradient of the field between electrode and handgrip is greater then the mean potential gradient between the electrode and the grounded article to be coated. Consequently, the electrical charging of the atomized material is essentially independent of the potential gradients which vary as a function of the spacing of the article from the electrode and dependent upon the practically constant potential gradient of the reverse field between the electrode and the handgrip. In order to obtain a uniform coating operation there is required a minimum spacing of the article from the coating device, which generally amounts to 20 to 25 centimeters, and additionally, for the guiding of the charged material particles there is necessary a sufficiently intense transport field which requires the application of a higher high-voltage of for instance 70 kV and more at the electrode. This leads to difficulties as concerns the reverse field decisive for charging. Moreover, attaining an improved charging by means of a reverse field which is more intense relative to the transport field is already problematic in terms of the safety of the operator of the coating apparatus and the generally existing relevant regulations concerning maintaining minimum spacing of the reverse field do not permit an intensity which is necessary for optimum charging of the material, so that the improvement in the efficiency which can be realized in fact with a reverse field is confined within rather narrow limits.